A converter connected to an a.c. network, for example a converter included in a converter plant for high-voltage direct current, generates, by its principle of operation, harmonic currents on its a.c. side and harmonic voltages on its d.c. voltage side. In this context, in principle, only harmonics to the fundamental feequency of the a.c. network of the orders n=kp.+-.1 occur on the a.c. side and of the orders n=kp on the d.c. voltage side, p being the pulse number of the converter and k being a positive integer. Harmonics of other orders may also occur in power networks of this kind, caused by, for example, unsymmetries between the phases of the a.c. network.
To reduce the stresses on components included in the power network, and originating from the harmonics, and to fulfil the requirements made on the effect on the network and telecommunication disturbances, shunt-connected filters are therefore generally installed to limit the propagation of the disturbances in the power network. Harmonics of a lower order, for example those which correspond to k=1 and for 6-pulse converters also k=2, are generally filtered through filters tuned to these harmonics whereas harmonics of a higher order may be filtered through a high-pass filter. The filters are composed of passive components, and during the dimensioning it is also taken into consideration that the filters on the a.c. side are to serve as members for generating reactive power. In general, however, the requirements for generation of reactive power in a converter plant for high-voltage direct power result in the need to install one or more further high-voltage capacitor banks on the a.c. side. In certain cases, it may be necessary to install tuned filters and high-pass filters also on the d.c. voltage side of the converter. In a converter plant for high-voltage direct current, these filters and the capacitor banks constitute plant components which essentially influence the function, volume and cost of the plant.
The tuned filters are generally designed as series-resonance circuits, comprising capacitive, inductive and sometimes also resistive impedance elements, tuned such that, at one or more of the harmonic frequencies expected in the power network, they are to exhibit a purely resistive impedance.
In narrow-band filters also a small change of the reactance of an impedance element included in the filter may cause a considerable deterioration of the function of the filter. Such a change may, for example, be caused by a fault in one part of a capacitive impedance element. A known method for monitoring this type of fault is to sense a voltage occurring in the filter and a current flowing therethrough and to form the phase difference between the components of the voltage and the current of a frequency selected by means of a bandpass filter, corresponding to the harmonic or one of the harmonics to which the filter is tuned. This phase difference shall be zero when the filter is correctly tuned. A state signal formed in dependence on the phase difference is supplied to an alarm unit to release a fault alarm if the phase difference exceeds a predetermined value.
Variations in network frequency and drift in component values, caused by, for example, temperature variations or aging, however, mean that an exact tuning generally cannot be maintained although no direct faults occur in the filter. It has therefore been proposed to provide the filters with tuning units which allow an adjustment of the resonance frequency or frequencies of the filter via a control member which influences the reactive impedance or impedances of the filter. One known method in this connection is to form a state signal in the manner mentioned above, which is supplied to the control member for the purpose of influencing the reactive impedance of the filter such that the phase difference is minimized in magnitude. The tuning units may, for example, comprise a switchblade capacitor bank or a reactor which is controllable by means of a semiconductor connection. A problem in this connection is that conventional methods for phase angle measurement, which are based on measurement of the difference in time between two signals whose frequencies are known only approximately, are very sensitive to frequency deviations. This means that the methods mentioned are less suitable for use in filters with very narrow bands.
To detect a fault in a capacitor bank, the present capacitance thereof can be directly calculated by means of Ohm's law from known amplitude values of current and voltage and be compared with the nominal capacitance value of the capacitor bank. To obtain sufficient sensitivity, however, this method requires that the amplitude values mentioned can be determined with high accuracy.